1. Introduction
This invention relates to a device for contacting surfaces with fluids especially adapted for forcing fluids through apertures passing through a planar substrate. More particularly, this invention relates to a fluid treatment device that is especially useful for the fabrication of printed circuit boards where substantial and complete fluid contact with the walls of through-holes within the printed circuit board substrate is critical for void free metallization and hole wall adhesion.
2. Description of the Prior Art
Printed circuit boards have solid metallic circuits formed from a conductive material positioned on one or both surfaces of an insulating dielectric material. Where the printed circuit board has two conductive surfaces positioned on opposite sides of a single dielectric layer, the resulting circuit board is known as a "double-sided circuit board". To provide electrical connections between circuits on opposite sides of a double-sided circuit board, a hole is typically drilled or punched through the double-sided circuit board substrate, i.e., through two metallic layers and a dielectric layer. These holes are known in the art as "through-holes" or, following metallization, "plated through-holes". To accommodate multiple circuits in a single printed circuit board, multiple copper layers are sandwiched between layers of dielectric material to produce a board known as a "multilayer circuit board". Analogous to the double-sided circuit board, the multi-layer circuit board also uses metallized through-holes or "vias" or blind vias to provide electrical conductivity between circuit layers.
Various processes have evolved over the years for forming conductive pathways between circuit layers separated by a dielectric layer, i.e.--for metallizing the walls of the through-holes passing through the circuit board. Initially, when the double sided circuit board was first produced, mechanical means such as rivets or eyelet's were inserted into the through-hole to act as a conductive pathway. However, the use of rivets or eyelet's was labor intensive and the metallic pathways so formed proved to be unreliable during use.
Current methods for forming metallic pathways in printed circuit board manufacture involve chemical metallization of the walls of through-holes. The predominant method for metallizing through-holes is disclosed in U.S. Pat. No. 3,011,920 incorporated herein by reference. The principal steps involved in the practice of this method involve contact of the printed circuit board substrate inclusive of the through-holes with an aqueous electroless plating catalyst composition comprising a reduced palladium-tin colloid followed by contact with an accelerator and then with an electroless plating solution, typically a copper plating solution. Contact of the palladium-tin colloid with the walls of the through-hole results in adsorption of the palladium-tin colloid on the through-hole walls. Subsequent contact with the accelerator removes protective colloids and contact with the electroless copper plating solution results in metal deposition onto the adsorbed colloidal palladium particles.
The electroless metal deposition process described above involves multiple fluid treatment steps in addition to the principal steps of contact with a palladium-tin colloid and an electroless copper plating solution. The walls of the through-holes have to be pre-treated to enhance adsorption of the colloid onto the walls. Pretreatment may include the steps of contact with an organic solution to soften the walls of the through-holes, an oxidant to etch back or desmear the hole walls, a neutralizer to remove residues resulting from the step of oxidation, a conditioner to enhance adsorption of the colloid onto the walls, etc. As discussed above, following treatment with the palladium-tin colloid, the through-hole walls are often treated with an accelerator solution to dissolve protective colloid from the adsorbed palladium-tin colloid and thereby activate dormant catalytic sites. Following electroless metal plating, the deposit is often enhanced (thickened) by electrolytic metal deposition. Water rinses are used between each of the fluid treatment steps.
In each of the fluid treatment steps described above, complete fluid contact with all surfaces of the through-hole wall is essential to avoid voids or disruptions in the metal deposit over the walls of the through-hole. Voids are formed as a consequence of incomplete metallization of the hole walls often due to incomplete or inadequate fluid treatment in any of the above described pretreatment steps, especially in the treatment of high aspect ratio holes (large length to diameter ratio) now more frequently required for increased board densification. Voids can result in rejection of a printed circuit board or failure of an electronic device in which the circuit is used.
Recently, several methods for direct electroplating walls of through-holes have been developed. These methods do not rely upon an intermediate step of electroless metal deposition. In accordance with one such method, the palladium-tin colloid catalyst used for electroless metal plating is found to be sufficiently conductive for electrolytic plating when adsorbed onto the walls of the through-hole in sufficient concentration. This method is disclosed in U.K. Patent No. 2,123,036 incorporated herein by reference. An alternative to this method is disclosed in U.S. Pat. Nos. 5,017,742 and 5,207,888, also incorporated herein by reference. With respect to this latter method, the same palladium-tin colloid used for electroless plating is treated to convert the same to a sulfide to thereby enhance its conductivity.
A more recently utilized direct plate process for metallizing the walls of hole-walls employs dispersions of carbon black or graphite for the formation of a conductive coating. The use of graphite to form conductive coatings on through-hole walls is known and disclosed in U.S. Pat. No. 2,897,409 incorporated herein by reference. Current processes are disclosed, for example, in U.S. Pat. Nos. 4,619,741 and 5,389,270, each incorporated herein by reference. In accordance with the procedures set forth in these patents, a dispersion of carbon black or graphite is passed through the through-holes to form a coating of the dispersion on the hole-walls. The coating is dried to yield an adhesively bonded conductive layer of the carbon black or graphite which is sufficiently conductive for electroplating in a conventional manner.
As in electroless metal plating, all of the direct plate methods for metallization of through-holes require multiple fluid treatment steps and all require full contact of a treatment fluid with the hole walls for total coverage of the hole wall with metal during the plating step. This is especially the case using carbon black or graphite dispersions where defect free dried coatings are essential for void free metal deposits and hole wall adhesion.
Prior art methods for treatment of through-holes with treatment fluids have involved the use of sequential dip tanks. In accordance with these prior art methods, circuit board substrates are racked on a suitable rack and the racks are then lowered into a first dip tank containing treatment fluid, held in that dip tank for a suitable period of time, raised and immersed in the next dip tank for further treatment. This process is continued through a sequence of tanks to a final metal plating tank. The process is labor intensive because the boards have to be racked and then unracked and the immersion time in each tank may be lengthy. In addition, the process uses large volumes of chemicals resulting in the need for excessive waste treatment procedures. A recent alternative to the use of dip tanks involves the use of conveyors to convey the printed circuit board substrates through a series of fluid treatment chambers. The substrates may be conveyed on a vertical or horizontal track using opposing rollers to pass the substrates along a predetermined path. Horizontal processing is currently preferred. In each treatment chamber, the fluid is applied to the circuit board substrate and into the through-holes typically using spray flood or jet nozzles.
In conveyerized processing of printed circuit board substrates, difficulties are encountered with spray nozzles for a variety of reasons. For example, the spray nozzle atomizes the fluid treatment solution causing it to become aerated, which can result in oxidation of oxygen sensitive components in the treatment solution. Moreover, due to the high energy of the fluid as it emerges from the spray nozzle and its turbulent flow, the treatment fluid fails to contact all surfaces of the through-hole as is necessary for complete metal coverage. In addition, following spraying of the treatment solution into the through-hole, some treatment fluid remains entrained within the interior of the through-hole. The entrained fluid is not readily removed by water rinsing and can block the through-hole preventing contact of the entire hole-wall with subsequent fluid treatment solutions. Moreover, spraying and atomizing of the treatment fluid lowers its temperature below the recommended treatment temperature for the fluid. Temperature control is known to be of importance in the metallizing of through-holes in printed circuit board manufacture.
Fluid jets are potentially more advantageous than spray nozzles because they are directional. However, it has been found that fluid jets fail to overcome the problem of entrainment of fluids within the interior of a through-hole and temperature drop. Attempts have been made to overcome the problems described above by use of specific arrays of fluid jets such as illustrated in U.S. Pat. No. 5,289,639 incorporated herein by reference, but these methods have to a large extent been unsuccessful and substantially increase the cost of printed wiring board production and equipment.
The problems described above have been exacerbated by the industry movement to e printed wiring boards with higher aspect ratios. The aspect ratio of a through-hole is the ratio of the thickness of a board to the diameter of the through-hole. As the aspect ratio increases, the ratio of the length of the hole to its diameter increases. The longer the hole and the smaller its diameter, the more difficult it will be for a treatment fluid to penetrate the entire length of the hole and the more readily the hole will entrain treatment fluid within its interior as it passes through the full sequence of treatment stations.
An alternative to spray nozzles and jet nozzles is disclosed in U.S. Pat. No. 4,789,405, incorporated herein by reference. In accordance with the procedures set forth in this patent, a printed circuit board substrate is passed over a surge line which is formed by a nozzle arrangement under a transporting path normal to the transporting direction of the board. A fluid treatment solution is applied to the lower surface of the circuit board substrate from a nozzle in the form of a standing wave. The nozzle can be formed as a plain perforation member provided with slots or holes arranged symmetrically or asymmetrically. The standing wave of the treatment fluid is said to have the capability of penetrating small diameter through-holes but in practice has not solved the problems described above. Moreover, the nozzle design needed to form the standing wave requires a nozzle having a large surface area parallel to the planar substrate. The combination of a nozzle of this design and the means required to convey the substrates over the nozzle consumes considerable space. A desire within the industry is to reduce the space required for a plating line.